Multiple resonant slot antenna



May 28, 1957 R. F. KoLAR :TAL 2,794,184

MULTIPLE' RESONANT SLOT ANTENNA v Filed July 21, 1955 Ernes J0/7/750/7TTORNEY I United States Patent. v.1()

MULTIPLE RESONANT SLOT ANTENNA Robert F. Kolar, Collingswood and Ernestt). Johnson Haddonfield, N. I., assignorsto Radio Corporation ofAmerica, a corporation of Delaware Application July 21, 1953, Serial No.369,348

12 Claims. (Cl. 343-767) This invention relates to a multiple frequencyslot antenna, and more particularly to an antenna having connectedthereto a single transmission line, the antenna being resonant at twodifferent frequencies unrelated to each other and also resonant atharmonics of the two frequencies.

ln radio communications, it is often desirable to be able to communicatevia any one of several different radio frequencies, the choice offrequency being made to select onev least aected by disturbing factorssuch as atmospheric conditions. It is very desirable that such acommunication system be capable of shifting between any one of thepredetermined frequencies without the necessity for making mechanicalmodifications or changes to lthe antenna structure itself. Whileremotely operated switches or the like may be mounted on an antenna,their use is to be avoided if possible because of the attendantmaintenance problems. vIt is, therefore, a general object of thisinvention to provide an improved antenna which is inherently resonant ataplurality of different frequencies.

-This invention relates to antennas of the type compricing a conductivesheet having an elongated slot therein and including a transmission lineconnected to opposite edges of the slot. Antennas of this type areusually referred to as slot antennas and they are especially useful athigh frequencies. The slot antenna lends itself to use on aircraft wherean airfoil of the aircraft serves as the conductive sheet and a slot cutin the airfoil serves as a means to apply or take off radio frequencyenergy. The slot may be filled with an insulating material to prevent aninterruption in the surface of the airfoil. It is often desirable thatan aircraft have radiol communicationsI equipment which is operable atany one of a number of different frequencies, and it is thereforeanother object of this invention to provide an improved slot typeantenna which is capable, without mechanical adjustment, of operation atdifferent frequencies. Y

It is a further object to provide an improved rriultifrequency slotantenna which may be unobtrusively in" corporated into a structure, suchas a building.

It is a still further object to provide an improved multifrequencyantenna which is characterized by its simplicity and its economy ofmanufacture.

In one aspect, the invention comprises a conductive sheet having anelongated rectangular slot therein. The sheet may be a flat sheet ofmetal, or it may be a conductive sheet formed or bent into any desiredshape. The slot in the conductive sheet is made to have a length suchthat the structure is resonant at the lowest frequency which it isdesired to transmit or receive. A transmission line is connected to theslot at a point intermediate the ends of the slot which provides theproper impedance match between the transmission line and the antenna.The antenna, as thus far described, is suitable for the reception of, ortransmission of, radio frequency energy at the frequency determinedbythe length of the slot and at frequencies which are odd harmonicsthereof. A stub 2,794,184 Patented May 28, 1957 is connected to the slotat a point between the point at which the transmission line is connectedand one end of the slot, and a second stub is connected to the slot at apoint between the point at which the transmission line is connected andthe opposite end of the slot. Each stub consists of two conductors,connected to opposite sides of the slot. The two conductors may be aquarter wave in length and open-circuited at the free ends, or may be ahalf wave in length and shorted together at the free ends. The stubshavelengths in terms of fractional wave lengths at the higher of the twofundamental frequencies which it is desired to transmit or receive. ofthe stubs is to shorten the effective length of the slot so that theantenna also resonates at the higher frequency. The antenna is thereforeoperative at either of the two fundamental frequencies which areharmonically unrelated, and at odd harmonics of the two fundamentalfrequencies.

These andother objects and aspects of the invention willnbe apparent tothose skilled in the art from the following more detailed descriptiontaken in conjunction with the drawing, wherein:

Fig. l is a perspective view of a multi-frequency slotin-plane antennaof the invention utilizing quarter wave length transmission line openstubs; and

Fig. 2 is a perspective view of a multi-frequency slotin-plane antennaof the invention utilizing half wave length transmission lineshort-circuited stubs.

In Fig. l, a conductive sheet 5, having a height H, and a width W, isconstructed of sheet metal, or wire mesh, or metallic paint laid upon asupporting structure. The dimensions H and'W are selected to provide thedesired polar pattern of the antenna. A rectangular slot 6 cut in theconductive sheet 5 has a length F and a width E. The width E of the slotmay be any conveniently small dimension, and the length F of the slot isequal to an experimentally determined value which provides resonance aat the lowest desired fundamental frequency of operation transmitting orreceiving apparatus.

far described is operative to radiate or receive radio fre-y of theantenna. The slot length F is between one-half wavelengthand onewavelength at the lowest desired fundamental frequency. The twoconductors of the transmission line feeder 7 are connected to oppositesides of the slot at a distance C from one end of the slot andY adistance D from the other end of the slot. The dimensions C and D aredetermined experimentally to provide the proper impedance match betweenthe transmission line 7 and the antenna. Line 7 extends to suitableradio TheV antenna thus quency energy at the lower fundamental frequencydetermined by the dimensions C and D of the slot and frequencies whichare odd harmonics thereof. i

An open section of transmission line 10, referred to as a shortingstub10 is connected to the slot 6 at a distance A fromthe point at which thetransmission line 7 is .connected. The stub 10 consists of two parallelconductors 11 and 12 extending from opposite edges of the slot.Conductors 11 and 12 are a quarter-wave in length at the higher of thetwo fundamental frequencies which it is desired to transmit or receive.A second similar transmission line stub 15, consisting of quarter-wavelong conductors 16 and 17, is connected to opposite edges of the slotsat a distance B from the point at which the transmission line 7 isconnected to the slot. Conductors 16 and 17 are likewise a quarter-wavein length at the higher of the two fundamental frequencies which it isdesired to receive or transmit. The distances A+B between stubs 10 and15 is experimentally determined at a value which makes the antennaresonant at the higher of the two fundamental frequencies, the distancebeing between one-half wavelength and one wavelength at the A higher ofthe two frequencies which it is desired to re- The effect ceive ortransmit. The relative magnitudes of the distances A and B are adjustedso that the part of the slot 6 between stubs 10 and 15 provides anVimpedance match with the tranmission line 7 at the higher of the twofundamental frequencies. The quarter-wave open stubs 10 and l5 operateto provide a short Ycircuit across slot 6 at the points on'the slotwhere they are connected'. The stubs provide a short circuit only at thehigher fundamental frequency and odd harmonics thereof. At this higherfundamenal frequency, and odd harmonics thereof, the antenna acts thoughtheslot dhas a length equal to A+B. At the lower of the fundamentalfrequencies, the shorting stubs 10 and 15 have very little, if any,practical eect on the operation of the antenna. The stubs 1t) and 15 actto add an insignificant amount of capacitance to the slot- 6 at thelower fundamental frequency.

It is'thus apparent that the antenna of Fig. l may be operated at eitherone of two fundamental frequencies and odd harmonics thereof without thenecessity'of switching or otherwise mechanically altering the antennalFig. 2 shows an alternative form of theinvention similar to that shownin Fig. 1 except that the shorting stubsV are in the form ofhalf-wavelength sections of transmissionv line shorted at the free endsthereof. The same numeral designations used in Fig. 1 have been used inFig. 2. for corresponding elements except that prime designation havebeen added. The shorting stub 10' consists of two conductors 11 and 12',the conductors being a halfwave in length at the higher of the twofundamental frequencies. The free ends of conductors 11 and 12' areconnected together. The half-wave shorted stub 10 serves to shortcircuit the slot at the point at which the stub is connected, so far asfrequencies equal to the higher of the two fundamental frequencies andodd harmonics thereof are concerned. The shorting stub 10 does not shortthe slot at other frequencies such as those equal to the lower of thetwo fundamental frequencies. A second half-wave shorted stub 15 is inall respects similar to the shorting stub 10.

The use of the half-wave shorted stubs 10 and'15' shown in Fig. 2 may beadvantageousV where it is ldesired to use such stubs for the additionalpurpose of supporting the conductive sheet 5 from a structure in spacedrelationship therewith.

By way of example only, an antenna was constructed according to Fig. 1with dimensions as follows:

44 Stubs and l5 3 The transmission line 7 has a characteristic impedanceof 280 ohms. The dimensions C and D are such as to provide an impedancematch to the 280-ohm transmission line 7 at a fundamental frequency of200 megacycles. The dimensions A and B are such as to provideanimpedance match to the 280-ohm transmission line 7 at a higherfundamental frequency of 800 megacycles, The antenna provided animpedance match to the transmission line 7 at frequencies including 200megacycles, 650 megacycles, and 800 megacycles. The antenna was resonantat 650 megacycles, rather than at 600 megacycles, the third harmonic,because of the capacitance added by the shorting stubs 1i) and 15.

It is apparent that by the use of shorting stubs of the proper length,the effective length of the slot 6 may be altered for certainpredetermined frequencies only, and that the antenna may be used for anyone of a number of frequencies without the need for making anymechanical alteration to the antenna structure. it will be understoodthat the shorting stubs short-circuit the slot 6 at the predeterminedfrequencies only due their frequency selective properties, incontradistinction with mechanical shorting connections made directlyacross the slot. A direct mechanical short-circuit across a slot iseffective at all frequencies. It will be understood that the inventionis not limited to an antenna for use at two predetermined frequenciesand odd harmonics thereof, but that additional shorting stubs may beemployed to provide for operation at three or more fundamentalfrequencies and the odd harmonics thereof. lt will also be understoodthat one of the shorting stubs iii, 15 may be a quarter-wave open stuband the other may be a halfwave shorted stub. Under certain specialconditions of frequencies and transmission line impedance, a singleshorting stub is sufficient.

What is claimed is: n

l@ An antenna comprising a conductive sheet having an elongatedhslottherein, the length of said slot being such as to provide resonance at afirst predetermined frequency, means to effectively short-circuit saidslot at a ksecond higher predetermined frequency, said means effectivelyproviding a shorter slot resonant at said second predeterminedfrequency, and a transmission line connected to said slot at a positionwhich provides an impedance match' at both of said predeterminedfrequencies;

2. A multi-frequency antenna comprising a conductive sheet having a slottherein which provides resonance at a first predetermined frequency, andshort-circuiting transmission line stub means connected to said slot toeffectively shorten said slot and provide resonance at a second higherpredetermined frequency.

3. An antenna comprising a conductive sheet having an elongated slottherein, a transmission line connected to the edges of said slot at apoint intermediate its ends` providing an impedance match at a firstpredetermined frequency, and short-circuiting stub means connected tosaid slot to provide an impedance match at a second higher predeterminedfrequency.

4. A'multi-frequency antenna comprising a conductive sheet' having anelongated slot therein, said slot having a length which providesresonance at a rst predetermined frequency and at least oneshort-circuiting stub connected to said slot to make said slot resonateat a second higher predetermined frequency, said short-circuiting stubhaving a'length making it operative to short said slot only at saidvsecond higherpredetermined frequency and odd harmonics thereof.

5. A multi-frequency antenna'as defined in claim 4 wherein saidshortingfstub comprises two parallel conductors connected to oppositeedges `of said slot and having a length equal to aquarter-wave at saidsecond higher" predetermined frequency.

6. A multi-frequency antenna as defined in' claim 4 wherein said`shorting stub comprises two parallel conductors connected to oppositeedges of said slot and having a length equal to a half-wave at saidsecond higher predetermined frequency, and means connecting the remoteends of said conductors.

7.. A multi-frequency :antenna as defined in claim 4 wherein saidshortingstub comprises a quarter-wave open stub at said second higherpredetermined frequency.

8. A multifrequency antenna'as defined inclaim 4 wherein said shortingstub comprises a half-wave shorted stub at said second higherpredetermined frequency.

9. A multi-'frequency antenna comprising a conductive sheet having anelongated slot therein, a transmission line including two conductorsconnected to opposite edges of said slot at a point which provides animpedance match between the transmission lineand the slot at a firstpredetermined frequency at which said slot is resonant, aV rst shortingstub connected to said slot at a point between one end of the slot andthe point at which the line is connected, `and a second shorting stubconnected to said slot at a point between the opposite end of the slotand the point at which the line is connected, said shorting stubs beingconstructed to effectively shorten said slot solely at a second higherpredetermined frequency and odd harmonics thereof, whereby said antennais operative at frequencies equal to said rst and second predeterminedfrequencies `and odd harmonics thereof without the necessity of anyswitching or mechanical change in said antenna.

10. A multi-frequency antenna as defined in claim 9 wherein saidshorting stubs are quarter-wave open stubs at said second predeterminedfrequency.

11. A multi-frequency antenna :as defined in claim 9 wherein saidshorting stubs are half-wave shorted stubs at said second predeterminedfrequency.

12. An antenna comprising a conductive sheet having an elongated slottherein, the length of said slot being such as to provide resonance at afirst predetermined frequency, :and means resonant to a secondpredetermined frequency higher than said first frequency coupled acrosssaid slot to effectively short-circuit said slot at said secondfrequency, said means effectively providing a shorter slot resonant atsaid second frequency.

References Cited in the file of this patent UNITED STATES PATENTS Amy eta1. May 4, 1942 Lees et al. Mar. 24, 1953 OTHER REFERENCES

